Reenable AUTO_OBJ by default.

The problem with it was a bogus .OBJDIR in some cases where creation of
object directories were purposely not attempted, such as for 'make cleandir'
and in etc/ sub-directories. In these cases bmake would start with a
bogus .OBJDIR like etc/ due to MAKEOBJDIR being a dynamic value based on
.CURDIR, SRCTOP, and OBJTOP. OBJTOP would not yet be defined but is
during early src.sys.obj.mk. That file and auto.obj.mk both were not
modifying .OBJDIR unless they expected to create the objdir. Thus in
these cases the .OBJDIR was left as etc/* rather than fixed to the
proper .CURDIR.

The issues were fixed in r325404 and r325416. An assertion to avoid the
bad .OBJDIR was added in r325405.

Sponsored by: Dell EMC Isilon